84 lines
3.4 KiB
C
84 lines
3.4 KiB
C
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/*
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* Copyright (c) 2003, 2007-14 Matteo Frigo
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* Copyright (c) 2003, 2007-14 Massachusetts Institute of Technology
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
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*
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*/
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#include "ifftw-mpi.h"
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/* Return the radix r for a 1d MPI transform of a distributed dimension d,
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with the given flags and transform size. That is, decomposes d.n
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as r * m, Cooley-Tukey style. Also computes the block sizes rblock
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and mblock. Returns 0 if such a decomposition is not feasible.
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This is unfortunately somewhat complicated.
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A distributed Cooley-Tukey algorithm works as follows (see dft-rank1.c):
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d.n is initially distributed as an m x r array with block size mblock[IB].
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Then it is internally transposed to an r x m array with block size
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rblock[IB]. Then it is internally transposed to m x r again with block
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size mblock[OB]. Finally, it is transposed to r x m with block size
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rblock[IB].
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If flags & SCRAMBLED_IN, then the first transpose is skipped (the array
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starts out as r x m). If flags & SCRAMBLED_OUT, then the last transpose
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is skipped (the array ends up as m x r). To make sure the forward
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and backward transforms use the same "scrambling" format, we swap r
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and m when sign != FFT_SIGN.
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There are some downsides to this, especially in the case where
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either m or r is not divisible by n_pes. For one thing, it means
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that in general we can't use the same block size for the input and
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output. For another thing, it means that we can't in general honor
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a user's "requested" block sizes in d.b[]. Therefore, for simplicity,
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we simply ignore d.b[] for now.
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*/
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INT XM(choose_radix)(ddim d, int n_pes, unsigned flags, int sign,
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INT rblock[2], INT mblock[2])
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{
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INT r, m;
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UNUSED(flags); /* we would need this if we paid attention to d.b[*] */
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/* If n_pes is a factor of d.n, then choose r to be d.n / n_pes.
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This not only ensures that the input (the m dimension) is
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equally distributed if possible, and at the r dimension is
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maximally equally distributed (if d.n/n_pes >= n_pes), it also
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makes one of the local transpositions in the algorithm
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trivial. */
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if (d.n % n_pes == 0 /* it's good if n_pes divides d.n ...*/
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&& d.n / n_pes >= n_pes /* .. unless we can't use n_pes processes */)
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r = d.n / n_pes;
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else { /* n_pes does not divide d.n, pick a factor close to sqrt(d.n) */
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for (r = X(isqrt)(d.n); d.n % r != 0; ++r)
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;
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}
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if (r == 1 || r == d.n) return 0; /* punt if we can't reduce size */
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if (sign != FFT_SIGN) { /* swap {m,r} so that scrambling is reversible */
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m = r;
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r = d.n / m;
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}
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else
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m = d.n / r;
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rblock[IB] = rblock[OB] = XM(default_block)(r, n_pes);
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mblock[IB] = mblock[OB] = XM(default_block)(m, n_pes);
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return r;
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}
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